Adverse short- and medium-term respiratory effects of work at the World Trade Center (WTC) disaster site have been widely documented in Fire Department of the City of New York (FDNY) first responders and others. Our FDNY team first reported WTC cough syndrome, demonstrated that work-related exposures to the disaster site increased the odds of aerodigestive symptoms up to four years post-9/11, and showed that the prevalence of respiratory diagnoses, both self-reported and FDNY-physician diagnosed, remained elevated seven to nine years after working at the site. It remains unclear, however, whether these health effects occurred shortly after working at the disaster site or if new disease or respiratory symptoms occurred continually over subsequent years. Stated differently, it is possible that the exposure/response gradient we and others have observed for the cumulative incidence of disease and prevalent symptoms can be attributed entirely to higher incidence rates shortly after 9/11/2001, and beyond that time, there is no exposure response gradient for incident effects. We propose innovative statistical methods - parametric survival models with change points - to study the incidence of new onset obstructive airway disease (OAD) disease diagnoses and symptoms over the first ten years following WTC exposure, with the goal of determining the length of time that exposure response gradients are observed among exposed FDNY firefighters. This work will allow estimation of the length of time that a relatively short-term, high intensity exposure may be associated with incident respiratory illness.